1. Field of the Invention
This invention relates to a hybrid vehicle and, more particularly, to a hybrid vehicle driven by a motor and an engine as a driving force.
2. Description of the Related Art
Conventionally, hybrid vehicles having a driving system using both an engine and a motor have been provided.
Various types of hybrid vehicles have been proposed, for example, the hybrid vehicle of the series type in which electric power, generated by a generator driven by driving the engine, is converted to a direct current to charge a battery, and electric power from the battery is converted to alternating current to drive a driving motor. Another hybrid vehicle is the parallel type in which the vehicle is driven to simultaneously or selectively transmit driving force produced by the engine and the driving motor to an output shaft and is accelerated or decelerated by controlling mainly the output of the driving motor.
In the aforementioned hybrid vehicle, the engine is usually operated in a state of optimum efficiency regardless of the driving speed. Generally, the state of optimum efficiency of the engine is in a high-load area, so that, in the hybrid vehicle, the engine is usually driven in a high-load area.
For instance, in the case of the hybrid vehicle of the parallel type, when the driving force of the engine is greater than the necessary driving force for driving, the motor is used as the generator to be able to charge the battery with redundantly generated energy as regenerative electric power. Further, in the hybrid vehicle which is braking, the motor is used as the generator to charge the battery with regenerative electric power transferred from energy produced by decelerating the vehicle.
Further, when the brake is applied while the engine is driven, energy generated by the engine is required to be convertedto regenerative electric power, however, the battery has a limitation for the level of charge and, therefore, in some cases, neither regenerative electric power caused by engine energy nor regenerative electric power caused by energy produced by decelerating can be charged to the battery.
In order to increase energy efficiency yet reduce generation of redundant regenerative electric power, the engine may be cut out while regenerative electric power is generated when braking. However, there is a disadvantage in that auxiliary machinery supplied with driving force from the engine cannot be driven when the engine is stopped. Thus another disadvantage also occurs when the vehicle is driven by using only the driving motor.
In the hybrid vehicle of the series type, the engine is required to be cut out when not supplied from the generator with electric power in excess of the level of charge of the battery when sufficient charge can be obtained, therefore the auxiliary machinery cannot also be driven.
It is an object of the present invention to provide the hybrid vehicle capable of retaining engine speed to exceed a specified value at all times even when being driven with the engine cut-out.